Industrial power plants typically burn fossil fuels, such as fuel oils, to generate power. However, the exhaust gases resulting from the combustion of fossil fuels contain harmful pollutants, such as oxides of nitrogen, NO.sub.x (primarily NO and NO.sub.2), or sulfur, SO.sub.x (primarily SO.sub.2). These emissions have been linked with harmful effects such as acid rain. Accordingly, environmental regulations provide maximum limits on the concentrations of certain gases that may be present in the exhaust gases from various combustion processes.
Fuel oils are graded by the American Society for Testing and Materials (ASTM) according to their specific gravity and viscosity, #1 being the lightest and #6 the heaviest. #6 oil is relatively less expensive than the other grades of oil, so it is the least expensive to burn. However, #6 oil has a relatively higher content of sulfur and nitrogen, which results in a higher concentration of nitrogen oxides and sulfur dioxide in the exhaust gases from the combustion of the oil. The actual nitrogen ,or sulfur content varies somewhat within each grade of oil.
Systems for controlling NO.sub.x emissions have been employed to reduce pollution and meet environmental requirements. For example, systems have been devised to control the ratio of air to fuel during combustion to reduce the formation of NO.sub.x during combustion. Scrubbers to remove SO.sub.x may be placed in the exhaust stream.